1. Field of the Invention
This invention relates to noise detecting circuits and, more particularly, to such circuits by which dropouts in, for example, a video signal may be detected.
2. Description of the Prior Art
In video signal playback systems, such as optical disk apparatus, video information normally is recorded in digital form. In the event of signal drop-out during playback, digital techniques are used to simulate or compensate for the lost information. While dropout compensation techniques are known to those of ordinary skill in the art, they are not used unless and until a dropout is detected. A typical video recording system of the type that may be used with optical disk apparatus uses a frequency modulator to frequency modulate luminance, color difference, synchronizing and voice signal components. In one embodiment of a dropout detector, the omission or loss of one or more cycles of the FM signal is sensed as a reduction in the frequency of the reproduced video signal. If this reduced frequency falls below some threshold level, a dropout is indicated. However, if the frequency of the reproduced signal is too large, as may be due to irregular reflectivity of the video disk and which, nevertheless, should be interpreted as a dropout, this form of dropout detector does not operate satisfactorily. That is, dropouts which result in a substantial reduction in frequency may be detected, but dropouts which result in a substantial increase in frequency are not.
One embodiment of this type of FM dropout detector generates pulses in response to zero crossings of the reproduced FM signal. Another embodiment uses a "coring" circuit to produce pulses corresponding to the "core" of the FM signal. When these dropout detectors are used, a loss of one or more cycles in the FM signal is sensed either by a failure to detect zero crossings or a failure to detect a "core" of the FM signal.
Another type of dropout detector operates on the FM demodulated signal. The resultant composite video signal includes a synchronizing portion which, typically, includes a negative-going horizontal synchronizing pulse, and a video information portion which, typically, includes both luminance and chrominance components that are of generally positive polarity. Noise signals in the video information portion are detected by providing a whiter-than-white clipping level; and any signal which exceeds that clipping level is interpreted as a noise signal due to dropout. Similarly, noise signals in the horizontal synchronizing signal are detected by providing a negative clipping level greater than the maximum sync tip level that is expected. Any signal which exceeds this negative clipping level is interpreted as noise due to dropout.
While this technique relies upon a whiter-than-white clipping level and a negative clipping level to sense dropouts due to irregular reflectivity, dropouts which are present in the form of blacker-than-black pulses in the video information are not sensed unless such noise pulses are so large as to exceed the negative clipping level. Likewise, dropouts which produce positive-going noise pulses in the horizontal synchronizing pulse are not detected unless those noise pulses are so large as to exceed the whiter-than-white clipping level. As a consequence, there is a so-called dead band within which negative-going noise in the video information and positive-going noise in the synchronizing information are not sensed. Hence, dropout sensitivity is less than satisfactory.